This invention relates to compound-semiconductor integrated-circuit (IC) devices and, more particularly, to high-speed double-heterostructure bipolar transistor (DHBT) devices made in IC form.
DHBT devices are well known and have been proposed for use in a wide variety of important practical applications. Thus, for example, DHBT devices are considered attractive candidates for inclusion in high-speed optical communication and processing systems. In such systems, DHBT devices are well suited to be integrated with optical and/or opto-electronic devices.
It is also well known that the high-speed operation of a DHBT device is limited by its base-collector capacitance. And it is further known that a large component of the base-collector capacitance originates in the so-called extrinsic base region of the device.
Straightforward attempts to reduce the base-collector capacitance of a DHBT device by, for example, simply reducing the extrinsic base area have not been satisfactory. This is so because reducing the extrinsic base area toward the minimum value permitted by the practical necessity to be able to form electrical contacts on that area increases the base resistance of the device. In turn, increasing the base resistance degrades the direct-current and microwave characteristics of the device.
The base-collector capacitance of a DHBT device can also be reduced by simply increasing the thickness of the semiconductor material below the base region. But this also increases the transit time for carriers in the intrinsic region of the device, whereby the frequency response of the device is degraded.
Accordingly, efforts have continued by workers skilled in the art aimed at attempting to devise effective ways of improving the speed properties of a DHBT device by reducing its base-collector capacitance. In particular, these efforts have been focussed on trying to reduce the base-collector capacitance in the extrinsic base region without deleteriously affecting other properties of the device. It was recognized that these efforts, if successful, could increase the speed of operation of high-performance DHBT devices (as characterized by the unity current-gain cutoff frequency and the maximum oscillation frequency) and thereby increase the liklihood that such devices would be used as components in a wide array of very-high-speed applications.